The field of the invention relates generally to Continuous Positive Airway Pressure (CPAP) devices. More specifically, the field of the invention relates to pressure regulating valves used in connection with CPAP devices.
CPAP devices are effective in treating obstructive sleep apnea. In all CPAP devices a mask (which may be nose mask, mouth mask, or face mask) is connected to a source of pressurized gas (typically air) via a flexible tubing. The source of gas is typically a flow generator that uses a turbine or blower that is connected to an electrically powered motor. During operation, the mask is then worn by the patient and the flow generator is powered to produce a positive mask pressure within the range of about 3 cm H2O to about 20 cm H2O. The positive applied pressure eliminates the negative pressure within the pharyngeal lumen thereby acting as a pneumatic splint to maintain the patient""s airway patency.
In CPAP therapy, the pressure of the delivered gas is carefully chosen by the sleep therapist or other qualified health care professional to maintain adequate airway pressure (i.e., maintain an open airway). This pressure can be different for each patient. It is therefore extremely important to accurately maintain the prescribed gas pressure during the administration of CPAP to the patient. If the pressure falls below the optimal prescribed pressure, the patient will often have an increased number of apneic or hyponeic episodes. In contrast, if the pressure rises above the optimal prescribed pressure, the patient will often times experience discomfort because of the increased work of breathing needed to overcome the high positive pressure.
In prior art CPAP devices, the pressurized gas is typically provided by a flow generator consisting of an electrically powered motor that is coupled to a turbine or impeller. Holes are incorporated into the patient mask to ensure a continuous flow of air thereby minimizing the rebreathing of exhaled gas by the patient. During inhalation, the flow rate of the air within the patient breathing circuit increases, which, in turn causes a commensurate decrease at the mask end of the breathing circuit. This decrease in pressure is below the optimal prescribed pressure. Conversely, during exhalation, the pressure experienced by the patient increases to a level that is higher than the prescribed optimal pressure.
Some prior art CPAP devices attempt to correct these transient pressure fluctuations during patient inhalation and exhalation by adjusting the speed of the motor that powers the flow generator. Typically, one or more pressure sensors are located within the patient breathing circuit and connected, in a feedback arrangement, with a controller that controls the speed of the motor powering the flow generator. When a pressure decrease is detected by the pressure sensor, the controller increases the speed of the motor to increase the flow rate within the breathing circuit (and thus increase the pressure therein). Similarly, when a pressure increase is detected by the pressure sensor, the controller slows the rotational speed of the flow generator motor to compensate for the pressure increase.
Unfortunately, this feedback arrangement has its limitations. Due to the relatively high inertia of the motor and the turbine/impeller, it is extremely difficult to develop a compensation system that has a rapid response time that can compensate for the transient pressure increases/decreases caused by the active breathing of a patient. The desire to improve the response time often results in oscillations in the output pressure of the flow generator. A survey of the performance of commonly used CPAP devices indicates that pressure fluctuations within +/xe2x88x922 cm H2O or higher can result within physiologic flow rates (i.e., between about 50 liters/minute to about 60 liters/minute).
U.S. Pat. No. 4,655,213 issued to Rapport et al. discloses a method and apparatus for the treatment of obstructive sleep apnea. The apparatus includes a nose mask assembly that is adapted to be sealed over the nose of a patient. The mask has an inlet for supplying continuous positive pressure of air to the mask. The mask also includes a threshold valve that releases air from the mask. While the apparatus of Rapport et al. is useful for relatively high pressures, the valve mechanism has serious limitations at low pressures. For example, in the Rapport et al. device, when the output pressure of the compressor falls below the threshold pressure of the valve, when the patient exhales, the transient increase in pressure is not enough to open the threshold valve. Consequently, the patient exhales CO2 laden gas into the breathing circuit. On the next inhalation, the patient re-breathes this exhaled gas. Serious health problems can result if this expired gas is not vented to atmosphere immediately upon exhalation.
Consequently, there is a need for a device and method that will deliver a prescribed air pressure to a patient receiving CPAP therapy. The device and method will be able to provide a substantially constant positive airway pressure to the patient and compensate for the transient pressure fluctuations associated with inhalation and exhalation. In addition, the device and method will permit the immediate evacuation of exhaled gases, even at very low pressures.
In a first aspect of the invention a valve for used in a CPAP device is disposed at a patient mask or, alternatively, at a location between the patient mask and the source of positive airway pressure. The valve includes a valve body having a first exhaust flowpath and a second exhaust flowpath. A floating valve seat is disposed in the valve body and is moveable between a first position and a second position. A moveable spring-biased piston provided in the valve and is releasably engaged with the floating valve seat. When the floating valve seat is in the first position, the moveable spring-biased piston is disengaged from the floating valve seat and gas travels out of the first exhaust flowpath. When the floating valve seat is in the second position, the moveable spring-biased piston is engaged with the floating valve seat and gas travels out the second exhaust flowpath.
In a second aspect of the invention, a mask is connectable to a source of positive airway pressure. A valve is disposed in the mask for controllably releasing gas therefrom to produce a substantially constant pressure inside the mask. The valve includes a valve body having a first exhaust flowpath and a second exhaust flowpath. A floating valve seat is disposed in the valve body and is moveable between a first position and a second position. A moveable spring-biased piston provided in the valve and is releasably engaged with the floating valve seat. When the floating valve seat is in the first position, the moveable spring-biased piston is disengaged from the floating valve seat and gas travels out of the first exhaust flowpath. When the floating valve seat is in the second position, the moveable spring-biased piston is engaged with the floating valve seat and gas travels out the second exhaust flowpath.
In a third, separate aspect of the invention, a method of delivering continuous positive airway pressure to a patient includes the steps of providing a source of positive airway pressure, providing a mask that connects the patient to the source of positive airway pressure, the mask including a valve therein for controllably releasing gas from the mask so as to produce a substantially constant pressure within the mask. The valve in the mask includes first and second exhaust flowpaths, wherein gas is released through the first exhaust flowpath in the valve when the pressure inside the mask exceeds the a threshold value and wherein gas is released through the second exhaust flowpath when the pressure inside the mask falls below the threshold value.
It is an object of the invention to provide a valve for use in CPAP devices that produces substantially constant pressure in the mask worn by a patient. It is a further object of the invention to provide a valve with a fail-safe feature that allows the patient to inhale and exhale atmospheric gas when the pressure within the mask worn by the patient falls below a threshold value. Additional objects of the invention are described in more detail below.